Electrolytic capacitors typically have a larger capacitance per unit volume than certain other types of capacitors, making them valuable in relatively high-current and low-frequency electrical circuits. One type of capacitor that has been developed is a “wet” electrolytic capacitor that includes a sintered tantalum powder anode. These tantalum “slugs” have very large internal surface area. These tantalum slugs first undergo an electrochemical oxidation that forms an oxide layer coating acting as dielectric over the entire external and internal surfaces of the tantalum body. The anodized tantalum slugs are then sealed in cans containing a highly conductive and generally corrosive liquid electrolyte solution, having high surface area with conductive linings allowing the flow of the current to the liquid electrolyte solution. Unfortunately, such wet capacitors can experience problems when the liquid electrolyte leaks. For example, gases (e.g., hydrogen) may be evolved during operation, causing pressure to build inside the capacitor. This may cause leaks to occur around conventional non-hermetic polymeric seals, where terminal wires protrude from the capacitor casing.
In light of the above, a gas-tight hermetic seal (e.g., glass-to-metal seal) is often employed through which the terminal wire can safely extend. Still, the hermetic seal itself can sometimes become corroded by the liquid electrolyte. For this reason, a liquid seal is also generally employed to prevent exposure of the inner region of the hermetic seal to the electrolyte. U.S. Pat. No. 7,206,186 to Knight, et al., for instance, describes a liquid seal that is formed by compressing elastomeric rings between the underside of the lid and a terminal plate connected to the capacitor element. A bushing may also be positioned inside the elastomeric rings to center the rings relative to the hermetic seal. Despite attempts at improving the liquid sealing of such electrolytic capacitors, problems nevertheless remain. For example, even when liquid seals are used, the present inventors have discovered that such capacitors still tend to exhibit poor equivalent series resistance (“ESR”) stability in high temperature environments associated with many commercial applications.
As such, a need still exists for an improved sealed wet electrolytic capacitor.